(a) Field of the Invention
The present invention relates to a transceiver for a base station with a smart antenna and a beamforming method in downlink.
More specifically, the present invention relates to a transceiver for a base station with a smart antenna for transferring signals from the base station to a mobile station, and a beamforming method in downlink in a wireless communication system using an antenna array or the smart antenna.
(b) Description of the Related Art
A mobile communication system makes it possible to communicate any kind of data with certain parties anywhere and at any time at all. To perfectly embody the mobile communication system, a next generation mobile communication system is commercialized which works based on a single reference protocol standardized all over the world and makes it possible to provide a far better service.
The next generation mobile communication system transmits and receives video and other data in addition to the speech data being serviced at present. Further, as the services become more varied, the bandwidth of transceiver data uses far wider band than the bandwidth at present. Thus, this would make demand for the mobile communication network increase.
Thus, the main objective of the next generation mobile communication system is to provide a method using as narrow a bandwidth as possible, and reliably transferring more data.
However, there are no known methods satisfying both reduction of the bandwidth and increase of reliability at the same time. Thus, the known methods can not solve both a capacity problem and a reliability problem arising with the next generation mobile communication system.
In recent years, a new method has been actively studied, which can satisfy both increase of capacity in the communication system and improvement of reliability. The new method may restrict interference and noises by controlling beam pattern. The new method is referred to as ‘Smart antenna technology’, and is one of the main technologies for the next generation mobile communication system.
In the smart antenna method, a base station sets a best suited beam to a subscriber of the wireless communication terminal, and this smart antenna method can reduce radio wave interference, increase capacity of the communication and improve the quality of the communication.
For example, the smart antenna system built at the base station actively meets the speed of targets including 1) a fixed target such as offices, 2) a target moving at low speed such as humans and satellites, and 3) a target moving at fast speed such as vehicles and trains; and continuously provides the best suited beam pattern. The smart antenna system provides best gain to the direction of the target and provides relatively small gain to the other directions to achieve restriction of interference. That is, the smart antenna system may increase capacity of the mobile communication system and improve the communication reliability.
Therefore, the smart antenna will be applied to the next generation communication method such as W-CDMA and CDMA 2000 etc. which are required to transfer many data reliably.
Meanwhile, when the smart antenna is applied to the base station for the wireless communication circumstance, the adaptive beamforming method and switched beamforming method can be considered as methods applicable in downlink or uplink.
The adaptive beamforming method forms a beam by continuously and adaptively changing a beam weight vector in accordance with circumstances where channels are changed.
On the other hand, the switched beamforming method has a predetermined number of beams and predetermines the switched beam weight vector. The switched beamforming method changes a switched beam index 120 and forms a beam in accordance with an antenna array direction of the mobile station as shown in FIG. 1. FIG. 1 shows a switched beam pattern in downlink according to the traditional method.
Generally, the adaptive beamforming method is widely used at the uplink base station. However, a time division multiplexing (TDD) method and a frequency division multiplexing (FDD) method are considered to determine the beamforming method. The adaptive beamforming method is widely used, using the uplink adaptive beam weight vector in downlink, when the time division multiplexing (TDD) method is applied, which frequencies are the same in downlink and uplink.
However, since frequencies in downlink and uplink are different in the frequency division multiplexing (FDD) method, the adaptive beam weight vector in uplink can not be applied in downlink. Here, the mobile station measures a channel in downlink and feedbacks the channel information to the base station. Thus, the adaptive beamforming or the switched beamforming methods are used in downlink.
That is, the traditional method receives the feedback information from the terminal and performs beamforming in downlink. In addition, the other method estimates DOA of receiving signals regardless of a demodulator and uses the DOA for beamforming in downlink.
The mentioned method that the mobile station measures the channel information and feedbacks the channel information to the base station and applies the channel information to the beamforming, can be effective, when the characteristics of the channel are good or the speed of the mobile station is low. However, errors can occur at the channel during feedback of the channel information or the performance can fall by feedback delay etc., when the characteristics of the channel are bad or the speed of the mobile station is high.
Further, since the mobile station continuously measures the characteristics of the channel in downlink and feedbacks the characteristics of the channel to the base station, the mobile station may become complicated. Further, since the feedback information is loaded in uplink, the capacity in uplink is reduced.
Meanwhile, as shown in FIG. 2, there is another method performing the switched beamforming in downlink. FIG. 2 shows a construction of a transceiver for determining the switched beam index in downlink using the search result by a switching beamforming searcher in uplink according to the traditional method. The detailed operation for each block will be described with reference to FIG. 3.
The transceiver of the base station shown in FIG. 2 includes a switching beamforming multi-channel searcher 260. The switching beamforming multichannel searcher selects a beam index of which receiving signal energy is greatest. The beam index is used as a switched beam index in downlink. In the transceiver of the base station, since the switching beamforming multi-channel searcher 260 searches all switched beam patterns, the complexity of the switching beamforming multi-channel searcher 260 increases along the number of the switched beam. For example, when a search range with ½ chip resolution is 64 chips in the W-CDMA mobile communication system, 128 correlators are required for the general searcher. However, 128×12 correlators are required for the switching beamforming multi-channel searcher 260 forming 12 beams.
Meanwhile, the U.S. Pat. No. 5,634,199 (filed in Apr. 17, 1995) discloses an invention entitled “Method of Subspace Beamforming using Adaptive Transmitting Antennas with Feedback”. The invention relates to a method for finding channel information in downlink and a method for finding a beamforming weight in downlink in the CDMA mobile communication system.
In detail, according to the U.S. Pat. No. 5,634,199, the base station transmits several probe signals and the terminal finds a correlating matrix for the probe signal and feedbacks the correlating matrix to the base station. Thus, the amount of the feedback data from the mobile station to the base station can be reduced. Then, the base station calculates the beamforming weight in downlink by using the correlating matrixes feedbacked from each terminal. However, since the invention uses the adaptive beamforming method using the feedback information from the mobile station in downlink, the performance can fall, when the characteristics of the channel are bad.
Meanwhile, the Korean patent application No. 2000-67975 (filed in Nov. 16, 2000) discloses the invention entitled “Downlink beamforming system using uplink array response vector and method thereof”. The invention provides a downlink beamforming system for forming an antenna beam with maximum gain to the direction the user is located.
In detail, according to the Koran patent application No. 2000-67975, two methods are proposed for forming a beam in downlink by using the received array response vector estimated in uplink. The first method relates to an algorithm that estimates the DOA of receiving signals and forms the maximum antenna gain to a desired user's direction, and the method increases the speed for estimating DOA. The second method estimates the received antenna response vector by using the received array response vector. The transmitting beamforming weight vector can be directly found by multiplying the received antenna response vector by a response conversion matrix. Thus, the antenna beam with maximum gain to the direction the user is located, can be formed. However, the complexity of the base station is not considered in the invention.
Meanwhile, the document entitled “Semi-Blind Method for Transmit Antenna Array in CDMA System” in VTC2000, 1st volume, pages 189-194, autumn 2000 discloses a method for finding downlink channel information by using feedback from the terminal and a method for finding beamforming weight downlink by using the downlink channel information, in the CDMA mobile communication system using the FDD method.
In detail, the document discloses a method comprising taking only uplink data, estimating an array response vector from the uplink data, receiving only the fast fading component as feedback from the terminal and calculating a downlink beamforming weight from the fast fading component. Thus, this method can solve the problem of an open loop beamforming method forming the downlink beam by using only uplink data. However, although this method can solve the problem of the pen loop beamforming method to some degree by using the downlink adaptive beamforming method, the complexity of the terminal or the base station can be increased.